Present day hunger for data and data storage has given rise to computing complexes in which multiple data processing systems have access to a single data warehouse that is often implemented by a complex of disk storage units. The growth of Internet use has fed this hunger, and added the requirement that the data be continuously available. In order to achieve this latter requirement, many database complexes and data warehouses resort to such techniques as “mirroring” (i.e. using redundant storage to maintain a copy of everything written to the main storage element), error correction of various types and the like. Redundant arrays of independent (or inexpensive) disks (RAID) is one example. Certain RAID configurations (“levels”) use data striping (spreading out blocks of each file across multiple disks) in order to protect the data, correcting errors when encounter, but redundancy is not used. This improves performance, but does not deliver fault tolerance. Other RAID levels (e.g., level 1) provide disk mirroring to add data redundancy and thereby fault tolerance.
While these techniques operate well to provide a measure of fault tolerance and, therefore, some continuous availability of the stored data, they can be overloaded when facilities employing these techniques must respond to a large volume of requests for data.
Thus, although continuous availability is now a feature of many database complexes and data warehouse configurations, they still can present a performance impediment by limiting the number of accesses that can be made at any particular moment in time.